In sailing, whether for long passages, short day-sails or for racing, it is important to be able to determine the best heading upwind. There do not appear to be any electronic devices, mechanical mapping tools or even standardized calculations available to plot the best route in advance. So whether in a large yacht or a small sailing dinghy, the skipper is left to their own judgment or trial and error about the heading. However, even experienced sailors have a difficult time picking the optimal sailing angle. Because the problem relies on trigonometry and speed calculations, there is nothing intuitive to help in determining the correct tradeoff. Smaller sailboats often do not even have a speedometer on board. Yet variations in the heading from the optimal course can add substantially to the arrival time, if the boat speed is halved to head closer to the target or if the distance is multiplied to get more speed off the wind.
What sail boats need is a convenient method of predicting Tacking Time to Destination (“TTD”) that takes into account the tradeoff between the boat's increasing speed as it heads off the wind and the increased distance. As we know, heading too far upwind shortens your route but slows down the boat. So the sailor needs to know the optimum route, both for racing and for long cruising passages, in order to get the best tradeoff between distance and speed.
Sailboat manufacturers sometimes create velocity prediction plots showing boat speed on different points of sail relative to the wind. However, this is only part of the information needed to determine the optimal Tacking Time to Destination. There are also tactical compasses that help to maintain the same angle to the wind when tacking. However, these units are expensive and require several boat tacks to calibrate with the wind, every time they are used. They also assume that the fastest way to get to a destination is simply to go as fast as possible, which is not always true. They are also limited to showing the sailing angle for maximal boat speed, rather than any distance information, travel time, or logic regarding minimizing the travel time to the destination.
Trigonometric tables may not have been widely available in the golden age of sail up to the 19th Century. With motorized shipping the issue of minimizing Tacking Time to Destination had less practical importance. Within the last decade, satellite phones have become widely accessible for web, email and telephone access at sea. Similarly the Global Positioning System (GPS) has become widely available beyond military applications and has displaced the sextant and celestial navigation for offshore cruising. Although trigonometric calculations can be made from GPS positions, current standards for navigation software and GPS chartplotters still seem to be designed for motorized naval vessels. GPS chartplotters assume that parameters like distance to destination and estimated time of arrival should be calculated as the crow (or seagull) flies, based on the shortest straight line to the destination.
GPS receivers and chartplotters offer a range of information and calculations for: location, elevation, estimated time of arrival (ETA), boat speed, distance to destination. These types of parameters can be displayed in a variety of interfaces showing steering down a road, a series of gauges, an aerial view of a chart, or a 3D scene model, for example. Unfortunately, for the sailor there is no way to get the best sailing angle from the current generation of GPS systems, and the straight-line Estimated Time of Arrival (ETA) has little value. One lesser-known parameter that is often available in GPS units is Velocity Made good (VMG). VMG assesses the component of speed towards the destination when tacking. However, VMG requires that the boat be driven in different directions to determine the best speed by trial and error. This is not practical and requires purposely heading in wrong directions. VMG also does not predict tacking time to destination or indicate the distances required. It is only a speed measure, not a heading that can be displayed and evaluated on an overview or chartplotter before a trip. A convenient system is needed to calculate and display the optimal route ahead of time, showing the angles, distances and travel time involved in different tacking routes. It should also be possible to define the optimal route, particularly where there is a counter-intuitive tradeoff such as a need to travel farther to arrive sooner (because of greater speed off the wind). Finally, it should be possible to determine and display the travel time, distance and heading angles for the current route in comparison to the optimal route.
For sailboats on passages, defining the optimal course and knowing how long it will take can have important consequences; for making landfall during daylight, for example. GPS chartplotters assume that the sailboat travels like a motorboat, straight along the rhumb line (i.e., the direct route to the target destination). However, sailboats have to tack back and forth when heading upwind (and sometimes when the wind is directly astern, as well). A method is needed by which sailors can address the long-standing navigational dilemma of how far off-course to head, to maximize speed off the wind. For sailboats heading upwind, there is no way of knowing how long different zig-zagging tacks will take or predicting this in advance.
In the long history of seafaring there has never been an easy method for determining which heading will lead to the fastest arrival time. What sailboats need is a “Tacking Time to Destination” (TTD) that takes into account the tradeoff between the boat's speed as it heads off the wind in relation to the increased distance. Then, a method of evaluating all of the possible routes is needed, to arrive at a decision of which is the optimal route to get to the destination with the shortest travel time. As we know, heading too far upwind shortens your route but slows down the boat. Conversely, heading off the wind will allow more speed, but at the cost of a much further distance to travel. So the sailor needs to know the optimum route, both for racing and for long cruising passages, in order to get the best tradeoff of a short route at a good speed. Ideally, a system is needed that can be taken on board that would quickly indicate the best heading to get to the destination fastest. This is the central navigating dilemma in sailboats of all sizes.